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CAS Number: 123-31-9 
EC / List no.: 204-617-8
Mol. formula: C6H6O2

Chemical formula: C6H6O2
Molar mass: 110.112 g·mol−1
Appearance: white solid
Density: 1.3 g cm−3, solid
Melting point: 172 °C (342 °F; 445 K)
Boiling point: 287 °C (549 °F; 560 K)
Solubility in water: 5.9 g/100 mL (15 °C)
Vapor pressure: 10−5 mmHg (20 °C)
Acidity (pKa): 9.9
Magnetic susceptibility (χ): −64.63×10−6 cm3/mol

Solubility of 1,4-Benzenediol
Water at 15°C......................................... 5 % w/w
20°C.......................................... 6 % w/w
30°C.......................................... 8 % w/w
40°C......................................11,5 % w/w
50°C........................................ 16 % w/w
Ethyl alcohol at 20°C............. freely soluble (56 % w/w)
Acetone at 20°C .................... freely soluble (44 % w/w)
Methanol ................................ freely soluble (1g in 3ml)
Ethyl ether ...................................... soluble (1g in 10ml)

1,4-Benzenediol, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. 
1,4-Benzenediol has two hydroxyl groups bonded to a benzene ring in a para position. 
1,4-Benzenediol is a white granular solid. 
Substituted derivatives of this parent compound are also referred to as hydroquinones. 
The name "hydroquinone" was coined by Friedrich Wöhler in 1843.

Preferred IUPAC name: Benzene-1,4-diol

Other names:
1,4-Hydroxy benzene

IUPAC names

1,4-Benzenediol, 1,4-Dihydroxybenzene, HQ, Quinol





1,4-dihydroxybenzene; hydroquinone; quinol

1,4-dyhydroxybenzene; hydroquinone, quinol

benzene, 1,4-dihydroxy











hydroquinone CAS information 


Product description
1,4-Benzenediol, Photographic Grade, is a white crystalline solid. 
HQ is used as a good general-purpose inhibitor, stabilizer, antioxidant, and intermediate. 

1,4-Benzenediol is offered in both photographic and USP grades for use in many applications.

1,4-Benzenediol is widely used as an inhibitor, an antioxidant, and as an intermediate chemical in the synthesis of dyes, motor fuels, and oils. In photographic processing, 1,4-Benzenediol is a phenol by-product with antioxidant possessions that causes toxicity in various organs, chiefly in the kidney.

1,4-Benzenediol is depleted for a topical treatment for skin hyperpigmentation and in distinct cosmetic products. Certain usage conditions comprise melasma, freckles, lentigines, age spots and acne scars. Skin sensitivity of 1,4-Benzenediol may be ascertained beforehand of the treatment by applying a small amount of cream to the hyperpigmented area.

Applications/uses of 1,4-Benzenediol:

Agriculture intermediates
Case and carton closures
Dyes / pigments / paints
Flame retardants
Graphic arts
Herbicides - intermediate for other
Kitchen & bath
Methyl methyl acrylate (mma)
Molding & trim interior
Paints & coatings
Peek (polyether ether ketone)
Photographic chemicals
Photographic supplies
Polymer modification
Polymer processing
Process solvents
Rubber modification
Specialty chemicals
Water treatment industrial

1,4-Benzenediol is a skin-lightening agent. 

1,4-Benzenediol bleaches the skin, which can be helpful when treating different forms of hyperpigmentation.

How does 1,4-Benzenediol work?
1,4-Benzenediol bleaches your skin by decreasing the number of melanocytes present. Melanocytes make melanin, which is what produces your skin tone.

In cases of hyperpigmentation, more melanin is present due to an increase in melanocyte production. 
By controlling these melanocytes, your skin will become more evenly toned over time.

1,4-Benzenediol is used to treat skin conditions related to hyperpigmentation. This includes:

acne scars
age spots
post-inflammatory marks from psoriasis and eczema

Although 1,4-Benzenediol can help fade red or brown spots that have lingered, it won’t help with active inflammation. 
For example, the ingredient can help minimize acne scarring, but it won’t have an effect on redness from active breakouts.

1,4-Benzenediol is a white, odorless, crystalline solid with an extremely low vapor pressure. 

1,4-Benzenediol is moderately soluble in water and highly soluble in alcohol. 

1,4-Benzenediol occurs in the environment as a result of anthropogenic processes, as well as in natural products from plants and animals. 

In the soil, 1,4-Benzenediol is expected to biodegrade under aerobic conditions. 
1,4-Benzenediol may be removed from the soil by oxidation processes or by direct photolysis on the surface. 
Volatilization would be minimal. In the water, it would degrade under either aerobic or anaerobic conditions. 
1,4-Benzenediol can also slowly oxidize to quinone, which is more volatile. 

In the air, 1,4-Benzenediol undergoes photochemical degradation. 

1,4-Benzenediol is listed as undergoing rapid biodegradation in a commercial activated sludge unit under aerobic conditions. 

The estimated and experimental bioconcentration factors for 1,4-Benzenediol of 40–65 have been obtained. 

These data indicate that 1,4-Benzenediol is not expected to significantly bioconcentrate in fish and aquatic organisms. 
1,4-Benzenediol, also, does not persist in the environment.

1,4-Benzenediol is a Polymerization inhibitor in the manufacturing of monomers like acrylics (AA, AE), methacrylics (MMA,MAA), AN, …

1,4-Benzenediol is an organic intermediate for agrochemical and may be used as starting raw material to be chemically transformed by pharmaceutical industries. 

As such the product is sold as a technical grade and is not produced under cGMP conditions.

Not to be used in cosmetics in the EU. 

Not to be used in cosmetic products as skin lightener in EU countries.

1,4-Benzenediol is used in the production of other chemicals. 

1,4-Benzenediol is also used in a variety of industrial and consumer adhesives, including as an additive to heat shrink tubing, restorative paste, bonding tape and as a reducing agent in photographic developing solutions.

1,4-Benzenediol is also used in certain cosmetic products such as manicure preparations and hair dyes.

1,4-Benzenediol is also used in some health products such as skin lightening preparations.

Uses of 1,4-Benzenediol:
1,4-Benzenediol has a variety of uses principally associated with its action as a reducing agent that is soluble in water. 

1,4-Benzenediol is a major component in most black and white photographic developers for film and paper where, with the compound metol, it reduces silver halides to elemental silver.

There are various other uses associated with its reducing power. 
As a polymerisation inhibitor, exploiting its antioxidant properties, 1,4-Benzenediol prevents polymerization of acrylic acid, methyl methacrylate, cyanoacrylate, and other monomers that are susceptible to radical-initiated polymerization. 
By acting as a free radical scavenger, 1,4-Benzenediol serves to prolong the shelflife of light-sensitive resins such as preceramic polymers.

1,4-Benzenediol can lose a hydrogen cation from both hydroxyl groups to form a diphenolate ion. 
The disodium diphenolate salt of 1,4-Benzenediol is used as an alternating comonomer unit in the production of the polymer PEEK.

1,4-Benzenediol is a polymerisation inhibitor used in process, transport and storage. 

1,4-Benzenediol is used for various applications such as: photographic developper, synthesis intermediate for rubber, dyes and for Agrochemicals and Pharmaceuticals industries.

1,4-Benzenediol is used as an oxygen scavenger in an aqueous medium in the water treatment industry.

Skin depigmentation
1,4-Benzenediol is used as a topical application in skin whitening to reduce the color of skin. 
It does not have the same predisposition to cause dermatitis as metol does. 
This is a prescription-only ingredient in some countries, including the member states of the European Union under Directives 76/768/EEC:1976.

In 2006, the United States Food and Drug Administration revoked its previous approval of 1,4-Benzenediol and proposed a ban on all over-the-counter preparations.
The FDA officially banned 1,4-Benzenediol in 2020 as part of a larger reform of the over-the-counter drug review process.
The FDA stated that 1,4-Benzenediol cannot be ruled out as a potential carcinogen.
This conclusion was reached based on the extent of absorption in humans and the incidence of neoplasms in rats in several studies where adult rats were found to have increased rates of tumours, including thyroid follicular cell hyperplasias, anisokaryosis (variation in nuclei sizes), mononuclear cell leukemia, hepatocellular adenomas and renal tubule cell adenomas. 
The Campaign for Safe Cosmetics has also highlighted concerns.

Numerous studies have revealed that 1,4-Benzenediol, if taken orally, can cause exogenous ochronosis, a disfiguring disease in which blue-black pigments are deposited onto the skin; however, skin preparations containing the ingredient are administered topically. 
The FDA had classified 1,4-Benzenediol in 1982 as a safe product - generally recognized as safe and effective (GRASE), however additional studies under the National Toxicology Program (NTP) were suggested in order to determine whether there is a risk to humans from the use of 1,4-Benzenediol.
NTP evaluation showed some evidence of long-term carcinogenic and genotoxic effects

While using 1,4-Benzenediol as a lightening agent can be effective with proper use, it can also cause skin sensitivity. 

Using a daily sunscreen with a high PPD (persistent pigment darkening) rating reduces the risk of further damage. 

1,4-Benzenediol is sometimes combined with alpha-hydroxy acids that exfoliate the skin to quicken the lightening process. 
In the United States, topical treatments usually contain up to 2% in 1,4-Benzenediol. 
Otherwise, higher concentrations (up to 4%) should be prescribed and used with caution.

While 1,4-Benzenediol remains widely prescribed for treatment of hyperpigmentation, questions raised about its safety profile by regulatory agencies in the EU, Japan, and USA encourage the search for other agents with comparable efficacy.
Several such agents are already available or under research, including azelaic acid, kojic acid, retinoids, cysteamine, topical steroids, glycolic acid, and other substances. 

One of these, 4-butylresorcinol, has been proven to be more effective at treating melanin-related skin disorders by a wide margin, as well as safe enough to be made available over the counter.[31]

1,4-Benzenediol (HQ) is the most commonly used tyrosinase inhibitor and is often the primary treatment option for postinflammatory hyperpigmentation and melasma.
The use of HQ results in the reversible inhibition of cellular metabolism by affecting both DNA and RNA production and reduces tyrosinase activity by 90%.

Although effective alone, HQ is often combined with other agents such as tretinoin, glycolic acid, kojic acid, and azelaic acid.

In the US, 1,4-Benzenediol is available over the counter in 2% formulations and by prescription in 4% formulations. 

1,4-Benzenediol is a derivative of benzene, which has prompted concerns about its safety. 

In 2001, European countries banned 1,4-Benzenediol for general cosmetic purposes, and its use is highly regulated in Asia. 

However, HQ has been used for more than 50 years under a doctor's supervision for medical conditions worldwide and there has not been a single documented case of either a cutaneous or internal malignancy associated with this drug.
The most serious adverse health effect observed in workers exposed to HQ is pigmentation of the eye and, in a few cases, permanent corneal damage.

The FDA is primarily concerned about the development of exogenous ochronosis, a disorder that paradoxically results in increased skin darkening, but only 30 cases of ochronosis have been attributed to the use of HQ in North America.

More common side effects of 1,4-Benzenediol include contact dermatitis and nail discoloration. 
Although most dermatologists agree that 1,4-Benzenediol is safe and is the most effective option, the safety debate about 1,4-Benzenediol within the FDA has spurred companies to research newer, less controversial skin lighteners.

Hydroquinone (HQ) has been the standard treatment of hyperpigmentation for many years. 
Its main mechanism of action is through the inhibition of tyrosinase, an enzyme necessary for the production of melanin, but it may also alter the formation of melanosomes and selectively damage melanosomes and melanocytes. 
Although very effective, it commonly causes skin irritation and may have a cytotoxic effect on melanocytes. 
Concern over cytotoxicity has resulted in the banning of HQ from use in skin products in some countries.

Hydroquinone may be helpful in epidermal-type melasma. 
Concentrations vary from 2% to 10% and hydroquinone may be used twice daily for 12 weeks. 

Hydroquinone may cause local skin irritation, however, and thereby leading to post-inflammatory hyperpigmentation, making the skin pigmentation worse. 
The patient should also be warned that if the hydroquinone happens to go onto surrounding normal skin, this may lighten as well and may give the patient leopard-skin appearance. 
Monobenzyl ether of hydroquinone, which is a permanent depigmentating agent, should never be used to treat melasma, as it causes irreversible loss of pigment. 
Exogenous ochronosis is also thought to be a rare side-effect of hydroquinone therapy.

Hydroquinone may be combined with topical tretinoin and 1% hydrocortisone in an ointment known as Kligman's solution, which may be applied once a day at night for a minimum of 4–6 months. 
There may be an irritant dermatitis about which the patient should be warned.

Azelaic acid may be used twice daily for 6 months and is tolerated very well, with very few side-effects.

More recent therapy for melasma has included glycolic acid peels, tretinoin peels and laser treatment.

Natural occurrences
Hydroquinones are one of the two primary reagents in the defensive glands of bombardier beetles, along with hydrogen peroxide (and perhaps other compounds, depending on the species), which collect in a reservoir. 
The reservoir opens through a muscle-controlled valve onto a thick-walled reaction chamber. 
This chamber is lined with cells that secrete catalases and peroxidases. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and catalyze the oxidation of the hydroquinones into p-quinones. 
These reactions release free oxygen and generate enough heat to bring the mixture to the boiling point and vaporize about a fifth of it, producing a hot spray from the beetle's abdomen.

Farnesyl hydroquinone derivatives are the principal irritants exuded by the poodle-dog bush, which can cause severe contact dermatitis in humans.

Hydroquinone is thought to be the active toxin in Agaricus hondensis mushrooms.

Hydroquinone has been shown to be one of the chemical constituents of the natural product propolis.

It is also one of the chemical compounds found in castoreum. 

This compound is gathered from the beaver's castor sacs.

In bearberry (Arctostaphylos uva-ursi), arbutin is converted to hydroquinone.

Production of Hydroquinone:
Hydroquinone is produced industrially by two main routes.

The most widely used route is similar to the cumene process in reaction mechanism and involves the dialkylation of benzene with propene to give 1,4-diisopropylbenzene. 
Hydroquinone reacts with air to afford the bis(hydroperoxide), which is structurally similar to cumene hydroperoxide and rearranges in acid to give acetone and hydroquinone.

A second route involves hydroxylation of phenol over a catalyst. 
The conversion uses hydrogen peroxide and affords a mixture of hydroquinone and its ortho isomer catechol (benzene-1,2-diol):
C6H5OH + H2O2 → C6H4(OH)2 + H2O

Other, less common methods include:

A potentially significant synthesis of hydroquinone from acetylene and iron pentacarbonyl has been proposed Iron pentacarbonyl serves as a catalyst, rather than as a reagent, in the presence of free carbon monoxide gas. 
Rhodium or ruthenium can substitute for iron as the catalyst with favorable chemical yields but are not typically used due to their cost of recovery from the reaction mixture.

Hydroquinone and its derivatives can also be prepared by oxidation of various phenols. 

Examples include Elbs persulfate oxidation and Dakin oxidation:
Hydroquinone was first obtained in 1820 by the French chemists Pelletier and Caventou via the dry distillation of quinic acid.

Reactions of Hydroquinone:
The reactivity of hydroquinone's hydroxyl groups resembles that of other phenols, being weakly acidic. 
The resulting conjugate base undergoes easy O-alkylation to give mono- and diethers. 
Similarly, hydroquinone is highly susceptible to ring substitution by Friedel–Crafts reactions such as alkylation. 
This reaction is exploited en route to popular antioxidants such as 2-tert-butyl-4-methoxyphenol (BHA). 
The useful dye quinizarin is produced by diacylation of hydroquinone with phthalic anhydride.

Hydroquinone undergoes oxidation under mild conditions to give benzoquinone. 
This process can be reversed.
Some naturally occurring hydroquinone derivatives exhibit this sort of reactivity, one example being coenzyme Q. 
Industrially this reaction is exploited both with hydroquinone itself but more often with its derivatives where one OH has been replaced by an amine.

When colorless hydroquinone and benzoquinone, a bright yellow solid, are cocrystallized in a 1:1 ratio, a dark-green crystalline charge-transfer complex (melting point 171 °C) called quinhydrone (C6H6O2·C6H4O2) is formed. 
This complex dissolves in hot water, where the two molecules dissociate in solution.

An important reaction is the conversion of hydroquinone to the mono- and diamine derivatives. 

Methylaminophenol, used in photography, is produced in this way:

C6H4(OH)2 + CH3NH2 → HOC6H4NHCH3 + H2O
Similarly diamines, useful in the rubber industry as antiozone agents, are produced similarly from aniline:

C6H4(OH)2 + 2 C6H5NH2 → C6H4(N(H)C6H5)2 + 2 H2O

Hydroquinone is a skin lightening agent available as either a pharmaceutical or a cosmeceutical. 

Its mechanism of action depends on its ability to inhibit tyrosinase synthesis, thereby inhibiting the production of melanin.
Other functions of hydroquinone include its ability to inhibit DNA and RNA synthesis, and to degrade melanosomes. 
Products sold at 2% concentration are available in more than 100 over-the-counter products, whereas those with a 3–10% concentration are prescription products and regulated as drugs. New products on the market today use hydroquinone in combination with topical retinoids and topical steroids for treatment of melasma and photopigmentation.

Hydroquinone has received scrutiny recently owing to its risk of ochronosis, a severe but rare side-effect. 
Endogenous ochronosis is a manifestation of a rare metabolic disorder known as alkaptonuria, which results from a deficiency of homogentisic acid oxidase. 
Exogenous ochronosis is a rare cutaneous side-effect of the long-term use of topical depigmenting agents such as hydroquinone. 
Ochronosis is characterized by an asymptomatic blue–black pigmentation of skin and cartilage. 
Although the exact cause of ochronosis from topical hydroquinone is not known, studies suggest that hydroquinone may inhibit homogentisic acid oxidase in the dermis, with the accumulation of homogentisic acid in the dermis causing ochronotic pigment deposition. 
Other agents reported in the literature to cause exogenous ochronosis are antimalarials, resorcinol, phenol, mercury, and picric acid.

A recent literature review revealed only 22 reported cases of ochronosis with hydroquinone use in more than 10 000 patient exposures over 50 years. 
This is an extremely low risk, and hydroquinone can be used safely in patients. 
Most cases of ochronosis reported are with long-term use of hydroquinone in doses greater than those in topical over-the-counter preparations. 
Although cases of ochronosis with the use of 2% hydroquinone have been reported, dermal absorption of hydroquinone up to 4% has been shown to be equivalent to that absorbed from ingestion of common foods containing hydroquinone. 
The risk of ochronosis is reportedly greater in African American women, when the product is used on large surface areas at concentrations greater than 4% for extended periods of time.

Hydroquinone at 2% concentration is widely used in topical cosmeceutical preparations. 
The current recommendations for its use are on hyperpigmented lesions for approximately 4–6 weeks. 
The benefits of hydroquinone are reportedly evident in the first 4–6 weeks of use, and plateau at 4 months. Use beyond 4 months is generally not recommended.

Combination products containing hydroquinone include the Kligman formula, which contains 5% hydroquinone with 0.1% retinoic acid and 0.1% dexamethasone in a hydrophilic ointment base. 
Newer products such as Tri-Luma® cream (galderma laboratories, Fortworth, TX) contain 0.01% fluocinolone, 4% hydroquinone, and 0.05% tretinoin.

The FDA has proposed warnings regarding the carcinogenic potential of hydroquinone. Hydroquinone, a metabolite of benzene, is an inhibitor of DNA and RNA synthesis. 
High doses of hydroquinone used for extended periods of time have been shown in laboratory animals to cause hepatic adenoma, renal adenoma, and leukemia. 
Allegations of hepatic and renal adenomas stem from murine studies and have not been reported in humans treated with oral or topical hydroquinone. 
Additionally, hydroquinone has been implicated in animal studies to cause mononuclear cell leukemia. 
The leukemogenic potential is only in the presence of phenol, and has been described in murine studies only after high-dose oral intake for over 2 years. 
No mononuclear cell leukemia has been reported with topical use of hydroquinone.

In conclusion, the FDA's proposed ban on the use of hydroquinone in over-the-counter preparations is based on carcinogen studies not validated in human trials. 
Ochronosis is, however, a documented side-effect of hydroquinone use and all patients should be advised against the long-term use of hydroquinone-containing products.

Hydroquinone (HQ) has been used for over 40 years to treat hyperpigmentation. 

It works by inhibiting the tyrosinase enzyme that converts tyrosine to 3–4 dihydroxy-phenylalanine (DOPA). 
Despite controversy over its use, many American dermatologic societies have come out with statements supporting the view that when used topically, this is a very safe ingredient. 
The controversy stems from studies in which oral ingestion of HQ was found to be carcinogenic in laboratory rodents. 
Another concern with long-term use of hydroquinone is the development of exogenous ochronosis. Ochronosis manifests as blue-black pigmentation of the skin due to the inhibition of homogentisic acid oxidase activity by HQ, resulting in the accumulation of homogentisic acid, which polymerizes to form ochronotic pigment in the dermis. 
The literature reports that this is mostly seen in native Africans (not African-Americans) and Latino patients or in patients using adulterated HQ products.
Many illegally imported creams have been found to be adulterated with mercury or other illegal ingredients. 

Mercury is a very toxic ingredient that has the ability to lighten skin.

Over-the-counter products contain 1–2% HQ, whereas concentrations above this are prescription strength. 
There are also combination products in which HQ is used with other agents that either enhance its penetration (AHA, retinol) or are synergistic in their skin lightening effects (vitamin C, topical steroids). 
HQ has a 12-h half-life and therefore the recommended application is twice a day. 
While it can be used to treat focal areas, it is often best to apply this agent to the whole face and apply a heavier amount over the darkest regions. 
This helps prevent a blotchy complexion. 

Patients should be reassured that contrary to popular opinion, hydroquinone does not bleach the skin but rather restores the skin to the patient's baseline skin color.

Skin lightening, or skin bleaching, is a cosmetic procedure that aims to lighten dark areas of skin or achieve a generally paler skin tone.

It's usually used to improve the appearance of blemishes such as birthmarks and dark patches (melasma).

Skin-lightening procedures work by reducing the concentration or production of melanin in the skin. Melanin is the pigment that gives skin its colour and helps protect it from the sun.

The main techniques used to lighten the skin include:

skin-lightening creams
laser treatment
Trying a skin-lightening procedure is a major decision. It can be expensive, time-consuming, and the results cannot be guaranteed.

If you're thinking of going ahead, be absolutely sure about your reasons for wanting to try it and do not rush into it.

It's a good idea to discuss your plans with a GP first. They might talk to you about your reasons for wanting to lighten your skin, and there might be a medical reason why the procedure is not appropriate for you.

Skin-lightening techniques can result in serious side effects and complications. 
People with dark skin tones are particularly at risk of these problems.

HYDROQUINONE belongs to a combination of 'skin cosmetics' primarily used for brighter, more radiant skin complexation. 
Besides this, it is also used to clears the complexion and acne scars, hyperpigmentation, melasma, and photoaging (premature ageing of the skin caused by repeated exposure to UV radiation). Hyperpigmentation is a skin condition in which patches of skin become darker in colour than the normal surrounding skin. Melasma is a common skin problem that causes dark, discoloured patches on your skin. Dark spots occur when some skin areas start producing more melanin (a natural pigment that gives colour to eyes, hair and skin), ranging from light to dark brown.

HYDROQUINONE contains Hydroquinone (skin lightening or bleaching agent) which works by decreasing the amount of melanin (a skin pigment) that is responsible for the darkening of the skin. HYDROQUINONE is primarily used to treat melasma (dark brown patch on skin), hyperpigmentation.

HYDROQUINONE is available in topical (for skin use) gel, cream, and emulsion forms. 
HYDROQUINONE is for external use only. 
Avoid contact with eyes, eyelids, lips, mouth, and nose. If the medicine comes in contact with any of these areas, rinse with water immediately. Do not use HYDROQUINONE on sunburned, windburned, dry, or irritated skin. Common side effects of HYDROQUINONE include dry skin, erythema (skin redness), burning sensation, mild itching, skin irritation. Most of these side effects of HYDROQUINONE do not require medical attention and gradually resolve over time. However, if the side effects persist or worsen, please consult your doctor.

If you are allergic to HYDROQUINONE or any other medicines, please tell your doctor. HYDROQUINONE can make the skin more sensitive in the sunlight, hence always use sunscreen and protective clothing before you step outdoors. Care should be taken while applying HYDROQUINONE on sensitive areas, such as the neck. Please limit the use of products that contain large amounts of alcohol (astringents, shaving creams, or after-shave lotions), hair removal products, and products containing lime or spices while using HYDROQUINONE. It is essential to let your doctor know if you are pregnant, planning to conceive, or a breastfeeding mother. HYDROQUINONE is not recommended for children below 12 years of age.

Melasma, Hyperpigmentation, Dark spots, Photoaging (premature aging of the skin caused by repeated exposure to UV radiation)

Medicinal Benefits
HYDROQUINONE contains ‘Hydroquinone’an skin-lighting agent used to treat skin problems like acne scars, melasma, hyperpigmentation, and photoaging. 
It also used to treat freckles (small brown spots on the skin), age spots and chloasma (darkened skin caused by hormonal changes). It works by decreasing the amount of melanin (a skin pigment) that is responsible for the darkening of the skin.

Directions for Use
HYDROQUINONE is available in topical (for skin use) gel, cream, emulsion. 

HYDROQUINONE is for external use only. Cream/Gel/Emulsion: Take the advised amount of gel/emulsion/cream with clean and dry hands on the skin's affected areas. 
Gently massage the medicine into the skin with your fingers. Wash your hands before and after applying HYDROQUINONE. Do not put dressing or bandage on the affected areas.

Store in a cool and dry place away from sunlight
Side Effects of HYDROQUINONE
Dry skin
Erythema (skin redness)
Burning sensation
Mild itching
Skin irritation
In-Depth Precautions and Warning

Drug Warnings
Do not apply HYDROQUINONE on open wounds or sunburned, windburned, dry, chapped, or irritated skin. 
If you are allergic to HYDROQUINONE or any other medicines, please tell your doctor. 

HYDROQUINONE is for external use only. 
Avoid contact of HYDROQUINONE with nose, ears, mouth, or eyes. 
In case HYDROQUINONE comes in contact with these areas accidentally, rinse with water thoroughly. 

If you are pregnant or a nursing mother, please inform your doctor before using HYDROQUINONE. 

HYDROQUINONE is not recommended for children below 12 years of age. 

HYDROQUINONE can make the skin more sensitive in the sunlight, hence always use sunscreen and protective clothing before you step outdoors. 

It is recommended to avoid tanning booths and sunlamps. 

Do not apply HYDROQUINONE in large amounts or use for a long time than prescribed as it does not give quick or better results but increases the risk of side effects. 
If you have asthma, rosacea (redness and often red, small, pus-filled bumps on the face), acne, skin thinning, perioral dermatitis (redness and swelling of the skin around the mouth), ulcerated skin, shingles (a viral infection causing painful rash), eczema (itchy, swelling of the skin) or any other skin condition, inform your doctor before taking HYDROQUINONE.

Hydroquinone is most commonly used in skin lighteners, products heavily marketed towards women of color. It is linked to cancer and organ-system toxicity.

FOUND IN: Skin lighteners, facial and skin cleansers, facial moisturizers, hair conditioners, finger nail coating products.

WHAT TO LOOK FOR ON THE LABEL: Hydroquinone or tocopheryl acetate

WHAT IS HYDROQUINONE? Hydroquinone is marketed most aggressively to women of color for its whitening ability in skin creams. The chemical is allowed in personal care products in the United States in concentrations up to two percent. Although banned in the European Union, a UK news report found that products containing hydroquinone were relatively easy to procure.[1]

In addition to its use in skin lighteners, hydroquinone is a possible impurity of tocopheryl acetate[2] (synthetic Vitamin E) which is very common in facial and skin cleansers, facial moisturizers and hair conditioners.

HEALTH CONCERNS: Cancer, organ-system toxicity, respiratory tract irritation.  MORE...

VULNERABLE POPULATIONS: Anyone is vulnerable to the effects of hydroquinone, but skin lighteners are marketed to women of color.

REGULATIONS: Banned from cosmetics in the European Union; restricted use in Canadian cosmetics. The U.S. Cosmetics Ingredients Review Panel indicates that hydroquinone is unsafe for use in products that are left on the skin, but because of lax enforcement, directions for skin-lightening products containing hydroquinone encourage frequent and consistent use on the skin.[7],[8],[9] It has been recommended to the National Toxicology Program (NTP) for further studies. The FDA continues to allow the availability of products containing hydroquinone in OTC drugs.

HOW TO AVOID: Consumers should avoid products that list hydroquinone. Consumers should check with companies to confirm the purity of tocopheryl acetate, due to concerns about hydroquinone contamination.



1,4-dihydroxybenzene; hydroquinone; quinol





Translated names
1,4- benzodiol (cs)

1,4-dihidroksibenzen (hr)

1,4-dihidroksibenzen (sl)

1,4-dihidroksibenzenas (lt)

1,4-dihidroksibenzols (lv)

1,4-dihidroxibenceno (es)

1,4-dihidroxibenzen (mt)

1,4-dihidroxibenzen (ro)

1,4-dihidroxibenzeno (pt)

1,4-dihidroxibenzol (hu)

1,4-Dihydroksibentseeni (fi)

1,4-dihydroksybenzen (no)

1,4-dihydroksybenzen (pl)

1,4-dihydroxibensen (sv)

1,4-dihydroxybenzeen (nl)

1,4-dihydroxybenzen (da)

1,4-Dihydroxybenzol (de)

1,4-dihydroxybenzène; hydroquinone; quinol (fr)

1,4-dihüdroksübenseen (et)

1,4-diidrossibenzene (it)

1,4-διυδροξυ-βενζόλιο (el)

1,4-дихидрооксибензен (bg)

benzén-1,4-diol (sk)

chinol (cs)

Chinol (de)

chinol (mt)

chinol (nl)

chinol (ro)

chinol (sk)

chinolis (lt)

chinolo (it)

hidrochinonas (lt)

hidrochinonă (mt)

hidrochinonă (ro)

hidrohinons (lv)

hidrokinon (hr)

hidrokinon (hu)

hidrokinon (sl)

hidroquinona (es)

hidroquinona (pt)

hinols (lv)

hydrochinon (cs)

Hydrochinon (de)

hydrochinon (nl)

hydrochinon (pl)

hydrochinón (sk)

hydrokinon (no)

hydrokinon (sv)

hydrokinoni (fi)

hydroquinon (da)

hydroquinone (fr)

hüdrokinoon (et)

idrochinone (it)

kinol (hr)

kinol (hu)

kinol (sl)

kinol (sv)

kinoli (fi)

kinool (et)

quinol (da)

quinol (es)

quinol (fr)

quinol (pt)

κινόλη (el)

υδροκινόνη (el)

хидрохинон (bg)

хинол (bg)

CAS names

IUPAC names

1,4-Benzenediol, 1,4-Dihydroxybenzene, HQ, Quinol





1,4-dihydroxybenzene; hydroquinone; quinol

1,4-dyhydroxybenzene; hydroquinone, quinol

benzene, 1,4-dihydroxy











hydroquinone CAS information ?


Trade names

Hydroquinone IG

Hydroquinone Photo

Hydroquinone Technical


Idrochinone IG

Idrochinone Photo

Idrochinone tecnico

Other names: 1,4-Benzenediol; p-Benzenediol; p-Dihydroxybenzene; p-Dioxybenzene; p-Hydroquinone; p-Hydroxyphenol; Arctuvin; Benzohydroquinone; Benzoquinol; Diak 5; Eldopaque; Eldoquin; Hidroquinone; Hydroquinol; HE 5; Phiaquin; Quinol; Tecquinol; Tenox HQ; 1,4-Dihydroxybenzene; 4-Hydroxyphenol; p-Dioxobenzene; Hydrochinone; Benzene, p-dihydroxy-; Black and White Bleaching Cream; Derma-Blanch; Hydrochinon; Hydroquinole; Idrochinone; NCI-C55834; Tequinol; USAF EK-356; 1,4-Dihydroxy-benzeen; 1,4-Dihydroxy-benzol; 1,4-Dihydroxybenzen; 1,4-Diidrobenzene; UN 2662; Dihydroquinone; Aida; Eldopacque; Eldopaque forte; Eldoquin forte; Solaquin forte; p-Dihydroquinone; Black & White Bleaching Cream; 1,4-Benzenediol (hydroquinone); Artra (Salt/Mix)

Hydroquinone was first synthesized in 1820 by Pelletier and Caventou by the dry distillation of quinic acid. 
In 1844, Wohler investigated the compound, established its structure, and named it hydroquinone. 
In the early 20th century, European patents were granted for its production by oxidation of phenol with alkaline permanganate (German) or with hydrogen peroxide (British). 
Another German method of preparation involved heating para-chlorophenol with copper sulfate under pressure. 
Hydroquinone was first used by Alonet in 1880 as a photographic developer, Merck and Company began producing hydroquinone in 1914. 
El du Pont de Nemours and Company was the first to manufacture hydroquinone in commercial quantities after World War I, using a process introduced by a French company, Usines de Rhone, This manufacturing process involved oxidation of aniline with sodium dichromate, reduction of the formed quinone with sulfur dioxide, and extraction of the hydroquinone with ether. 
In 1924, in an effort to reduce the cost of hydroquinone, Eastman Kodak decided to manufacture its own chemical products and formed a subsidiary, Eastman Chemical Corporation, which began operation that year. 
Aniline was oxidized to quinone by manganese dioxide and sulfuric acid, and the formed quinone was reduced with iron dust to hydroquinone. 
Besides these methods of synthesis, two other techniques for hydroquinone production from quinone have been patented. 
In one of these, quinone is distilled under vacuum into a solution of hydroquinone, at a temperature below 35 C, to form quinhydrone. 
The quinhydrone is subsequently reduced to hydroquinone. 
The second method involves the reaction of quinone in a substantially neutral, aqueous solution, in the presence of metallic zinc and a small quantity of potassium or ferric chloride, by heating at 70-75 C. Since hydroquinone can be easily oxidized to quinone and quinone-like products, it has become one of the most widely used organic reducing agents.

Hydroquinone (C6H4(OH)2), formula weight 110.11, is a white solid that can be crystallized from water as hexagonal prisms [1,4].

It has a melting point of 173-74 C and a boiling point of 285 C at 730 mmHg. 

Hydroquinone is soluble in water to the extent of 73 g/liter at 25 C. 
It is also highly soluble in alcohol and ether but only slightly soluble in cold benzene (about 0.2 g/liter) and in other nonpolar solvents. 

Hydroquinone possesses two phenolic hydroxyl groups having dissociation constants of 1.22 x 10*10 and 9.18 x 10*13, respectively, at 30 C (* means "to the negative power of"). 
The designation "hydroquinone" is given to 1,4-dihydroxybenzene and as a general designation, to 1,2- dihydroxybenzene and 1,3-dihydroxybenzene, which are given the more specific names of catechol and resorcinol, respectively.

The oxidation potential of hydroquinone at 20 C and pH 7.03 is 0.2982 volts. 
Hydroquinone has a very low vapor pressure (0.000018 mmHg at 25 C), while quinone sublimes at room temperature (0.1 mmHg at 25 C). 

The most common synonyms used for hydroquinone include p-dihydroxybenzene, 1,4- benzenediol, and 1,4-dihydroxybenzene. 
Most of the commercial uses of hydroquinone are related to its chemical property as a reducing agent. 
Hydroquinone and quinone form a reversible oxidation-reduction system, but the nature of this oxidation-reduction system is somewhat more complex than that shown in the above equation.
The formation of a relatively stable semiquinone radical by a single electron transfer to quinone has been reported. 
This semiquinone radical can undergo reversible dimerization reactions to form peroxides with other compounds or with quinone to form a colored "charge transfer" complex called quinhydrone. Autoxidation of hydroquinone to quinone proceeds in two steps.
In the first, a divalent hydroquinone ion loses one electron and yields a semiquinone ion, which gives off an electron to form quinone. 
The semiquinone ion is formed also by the reaction of a hydroquinone ion with quinone. 

Hydroquinone is easily oxidized to quinone by nitric acid, halogens, and persulfates, and, in alkaline solution, by oxygen. 
This reaction is reversible with a suitable reducing agent. 
However, in the dry form pure hydroquinone is quite stable, darkening slowly upon prolonged exposure to air. 
The oxidation of hydroquinone is very rapid in the presence of alkali, producing a brown solution when the substance is exposed to air. 
In an alkaline solution, hydroquinone is readily oxidized to quinone and hydrogen peroxide by oxygen . 
The peroxide oxidizes quinone to hydroxyquinone. 

The oxidation rate of hydroquinone by oxygen is rapid even in a slightly alkaline solution, and the reaction is strongly catalyzed by the cupric ion. 
Oxidation products included p-benzosemiquinone, hydroxy-benzoquinone, p-benzoquinone, and di-p-benzoquinone. 
Hydroquinone is also oxidized by Fehling's solution in the absence of air. 
It may prevent the oxidation of substances, such as aldehydes and sulfite solutions. 
Silver salts are rapidly reduced by hydroquinone at room temperature.
In acidic solution hydroquinone is very resistant to oxidation. 
The oxidation is slight until the solution becomes more alkaline than pH 7.3- 7.8, and then it becomes very rapid. 
Airborne hydroquinone in the occupational environment may be oxidized to quinone at room temperature in the presence of moisture. 
However, neither the rate of oxidation nor the equilibrium concentrations at room temperature are known. 

Hydroquinone is widely distributed in nature as a component of glucoside arbutin, from which it can be produced by hydrolysis. 
Arbutin can be obtained from the leaves of many plants; among the most important are the bearberry (Arctostaphylos uva ursi), the mountain cranberry (Vaccinium vitisidaea), the whortleberry (Vaccinium myrtillus), and the honeyflower (Protea mellifera). 
Arbutin is a minor constituent of aniseed oil from the fruit of the evergreen Chinese anise (Illicium verum),and it has been isolated from the bark and buds of the pear tree. 
Hydroquinone has also been found in cigarette smoke, Hydroquinone can be synthesized by the oxidation of aniline with sodium dichromate and sulfuric acid and subsequent reduction by bisulfite, by the reduction of quinone by nascent hydrogen liberated from a mineral acid by metal, by persulfate oxidation of phenol, or by the reaction of acetylene with carbon monoxide. 
The most popular and commercially useful method for the production of hydroquinone is the oxidation of aniline by manganese dioxide or sodium dichromate in sulfuric acid to quinone, which is subsequently reduced to hydroquinone with iron dust. 
Recently, another method has been developed to produce hydroquinone from propylene and benzene.
Benzene is alkylated with propylene to p- and diisopropyl benzene. 

The p-diisopropyl benzene is oxidized to the dihydroperoxide and then rearranged to yield hydroquinone and acetone. Nomiyama et al and Williams  reported that one of the metabolic (oxidative) products of benzene is phenol and that phenol is further metabolized to hydroquinone in rabbits and in humans. 
These metabolites of benzene are excreted mainly in the conjugated forms. 

In rabbits, ethereal sulfate formation appeared to be more important than glucuronic acid conjugation. 
Bakke reported that, when rats were fed 10% extra tyrosine in their diet, small amounts of hydroquinone (as much as 1.4-1.7 mg/24 hours for 2 days) were detected in the urine by gas and thin-layer chromatographic analysis. 
Gregg and Nelson reported that quinone was formed when hydroquinone was oxidized in the presence of the enzyme laccase, one atom of oxygen being used per molecule of hydroquinone. 

However, the same amount of hydroquinone was oxidized by tyrosinase in the presence of catechol, but more than one atom of oxygen per molecule of hydroquinone was consumed. The author concluded that the tyrosinase brought about a different type of oxidation than did the laccase, probably the introduction of a third hydroxyl group on hydroquinone 

Hydroquinone is used in several ways. 
Hydroquinone is used extensively as a photographic developer. 
Hydroquinone is used also as a dye intermediate and as an antioxidant and stabilizing agent. 

For instance, small amounts of hydroquinone greatly retard the autoxidation of furfural, formaldehyde, isopropyl ether, esters of linoleic acid, olefins, and ethylcellulose, Hydroquinone can be used to stabilize such compounds as epinephrine and furan against oxidation in solutions . Biologic specimens, such as phospholipid preparations, can be preserved by the addition of 0.5-1.0% hydroquinone. 
Ferricytochrome c can be reduced rapidly to the ferrous state by hydroquinone. 

The antioxidant activity of hydroquinone is also seen in its influence on polymerization reactions  where hydroquinone often inhibits polymerization by reacting with and destroying free radical intermediates.
Similarly, hydroquinone is used to retard the gelation of rubber sols and to prevent the precipitation of gums from leaded aviation gasoline. 
The rate of deterioration of cotton fabrics containing copper naphthenate and exposed to light can be decreased by hydroquinone. 
As a photographic developer, hydroquinone reduces the exposed silver halide grains in a photographic emulsion at a rate considerably faster than that of the unexposed grains. 
Sodium sulfite is added to developing solutions to prevent autoxidation of the developer, and quinone is immediately removed from the solution. 
Compared with other developing agents, eg, p-aminophenol, p-methylaminophenol, pyrocatechol, and pphenylenediamine, hydroquinone is distinguished by its energetic action, producing high densities of developed silver and images of high contrast.
 However, the developing action of hydroquinone is sensitive to the concentration of bromide ions in the developing solution. 

Many insects synthesize simple quinones, and these substances' high vapor pressure and unpleasant odor and taste serve as a defense against predators. 
Plants such as tobacco, alfalfa, and wheat contain a variety of more highly substituted quinones and their cyclization products, eg, tocopherols. 

Cultures of a number of bacteria produce quinones of biological significance, including Coenzyme Q. 
This coenzyme, also known as ubiquinone, operates as an electron acceptor in the metabolic process of oxidative phosphorylation in the mitochondria of mammalian cells. 
Naphthoquinones, such as vitamin K p are found in several plants, and vitamin occurs in putrefied fish meal. 
Biologically active quinones, eg, plastoquinone and ubiquinone (coenzyme Q ) , catalyze some biochemical reactions in animals, plants, and microorganisms. 
These substances appear to be of suitable size, shape, and redox activity to transfer electrons to and from other coenzymes. 
Quinones or phosphorylated quinols may play an active role in oxidative phosphorylation and in the respiratory cycle. 
Hydroquinone derivatives are also important antioxidants, although many of them are too toxic to be used in biologic systems . 
A hydroquinone derivative having "lipid properties" and the ability to be metabolized to nontoxic compounds would be a potentially useful antioxidant for edible fats. 
The antioxidant capacity of hydroquinone is superior to that of alpha-tocopherol 
Although hydroxychromans, tocopherols, and 5-hydroxycoumarans belong to the group of phenolic antioxidants, they are also derivatives of hydroquinone and form a separate subgroup. 
Some of the naturally occurring inhibitors from vegetable oils are similar to or even identical with tocopherols and may replace vitamin E, although the vitamin E-like activity does not parallel the antioxidative potency. 
At least eight naturally occurring tocopherols with both pregnancy-maintaining and antioxidative activities are known. Kusumoto and Nakajima found that hydroquinone did not produce methemoglobinemia in vivo but had a strong methemoglobin-forming action in vitro. 

However, quinone acts to form methemoglobin both in vivo and in vitro. 
Quinone also readily combines with proteins, probably by additional reactions involving free amino and sulfhydryl groups, and this property causes quinone to stain skin and accounts for its use as an agent for tanning leather. 
Cohen and Hochstein observed the generation of hydrogen peroxide in human erythrocytes after the addition of the hydroquinone-quinone redox system. 
This finding supports the concept that hydrogen peroxide toxicity plays a major role in hemolysis induced by exposure to hydroquinone.

Hydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound which is a type of phenol, having the chemical formula C6H4(OH)2. 
Its chemical structure, shown in the table at right, has two hydroxyl groups bonded to a benzene ring in a para position. 
It is a white granular solid at room temperature and pressure.

Hydroquinone is the name recommended by the International Union of Pure and Applied Chemistry (IUPAC) in its 1993 Recommendations for the Nomenclature of Organic Chemistry

Hydroquinone can undergo mild oxidation to convert to the compound parabenzoquinone, C6H4O2, often called p-quinone or simply quinone. Reduction of quinone reverses this reaction back to hydroquinone. Some biochemical compounds in nature have this sort of hydroquinone or quinone section in their structures, such as Coenzyme Q, and can undergo similar redox interconversions.

The hydroxyl groups of hydroquinone are quite weakly acidic. Hydroquinone can lose an H+ from one of the hydroxyls to form a monophenolate ion or lose an H+ from both to form a diphenolate ion.

Hydroquinone has a variety of uses principally associated with its action as a reducing agent which is soluble in water. 
Hydroquinone is a major component in most photographic developers where, with the compound Metol, it reduces silver halides to elemental silver.

In human medicine, hydroquinone is used as a topical application in skin whitening to reduce the color of skin as it does not have the same predisposition to cause dermatitis as Metol does. This use is banned in some countries (e.g. France) because of fears of a cancer risk.

The disodium diphenolate salt of hydroquinone is used as an alternating comonomer unit in the production of the polymer PEEK.

As a polymerization inhibitor, hydroquinone prevents polymerization of acrylic acid, methyl methacrylate, etc.

Hydroquinone is also used as a raw material of herbicides, rubber antioxidants and dye stuffs.

Natural occurrences
Hydroquinones are one of the two primary reagents in the defensive glands of bombardier beetles, along with hydrogen peroxide (and perhaps other chemicals, depending on the species), which collect in a reservoir. The reservoir opens through a muscle-controlled valve onto a thick-walled reaction chamber. 
This chamber is lined with cells that secrete catalases and peroxidases. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and catalyze the oxidation of the hydroquinones into p-quinones. These reactions release free oxygen and generate enough heat to bring the mixture to the boiling point and vaporize about a fifth of it, producing a hot spray from the beetle's abdomen.

Hydroquinone is produced as an inhibitor, an antioxidant, and an intermediate in the synthesis of dyes, motor fuels, and oils; in photographic processing; and naturally in certain plant species, Hydroquinone is a phenol derivative with antioxidant properties that can cause toxicity in several organs, notably the kidney. Used as a topical treatment for skin hyperpigmentation and in various cosmetic products, it is metabolized mainly to glutathione conjugates and forms mutagenic DNA adducts in in-vitro systems. (NCI04)

NCI Thesaurus (NCIt)
Hydroquinone appears as light colored crystals or solutions. May irritate the skin, eyes and mucous membranes. 
Mildly toxic by ingestion or skin absorption.

Hydroquinone is a benzenediol comprising benzene core carrying two hydroxy substituents para to each other. 
It has a role as a cofactor, a carcinogenic agent, an Escherichia coli metabolite, a human xenobiotic metabolite, a skin lightening agent, an antioxidant and a mouse metabolite. It is a benzenediol and a member of hydroquinones.

Uses of Hydroquinone
The uses of hydroquinone are:

As a reducing agent.
For the preventive measures of methyl methacrylate.
In skin whitening.
Helpful as a biomarker for benzene exposure.
By photographic developers
In the treatment of acne scars
In various cosmetic products
Applications of Hydroquinone
Hydroquinone has several applications, which are primarily associated with its function as a reducing agent that is soluble in water. 

It is a major component of most black and white photographers for film and paper where, with the compound metol, it transforms silver halides into elemental silver.
There are several other applications for its reducing power. 
As a polymerization barrier, hydroquinone inhibits the polymerization of acrylic acid, methyl methacrylate, cyanoacrylate, and other monomers vulnerable to radical-initiated polymerization by using its antioxidant properties.
By serving as a free-radical scavenger, hydroquinone helps in improving the shelflife of light-sensitive resins such as preceramic polymers. 
Hydroquinone can form a diphenolate ion by losing a hydrogen cation from both hydroxyl groups.


Boiler water chemicals
Corrosion inhibitors
Neutralizing agents
Oxygen scavengers
pH conditioners
Resin cleaners
Scale inhibitors

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